A 70 kg man has a hematocrit of 40 percent and an estimated blood volume of 5 liters. What is his approximate plasma volume?

• A 1.5 L
• B 2.0 L
• C 3.0 L ✓
• D 4.0 L

Explanation

Plasma volume equals blood volume multiplied by the fraction occupied by plasma, which is 1 minus the hematocrit. Here that is 5 L times 0.60, giving 3.0 L. A common error is multiplying by the hematocrit itself, which yields the red cell volume of 2.0 L and corresponds to distractor B. Note that the hematocrit slightly overestimates the true red cell fraction because trapped plasma between cells, so measured plasma volume by dye dilution runs marginally higher, but the calculated value remains the standard exam answer.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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